1. Field of the Invention
The present invention relates to a powder carrier screw which is suitable for carrying, collecting and agitating powder such as toner and developer for use in a development device and a waste toner mechanism in an image forming device forming a toner image such as a copier and a printer.
2. Description of the Related Art
A powder carrier screw has been conventionally used for carrying powder such as toner (for example, JP H11-223986A). This conventional powder carrier screw includes a rotation shaft, which rotates upon input of a rotation driving force, and a spiral portion which is wound about the rotation shaft in a spiral manner. The end portion of the spiral portion is joined to the rotation shaft by welding or soldering.
Regarding such a powder carrier screw, the present inventor invented a powder carrier screw OA (hereinafter, this is referred to as a prior powder carrier screw OA) as the first invention as illustrated in FIG. 8.
This prior powder carrier screw OA is applied to a toner carrier device OB. In this toner carrier device OB, as is known in the art, the powder carrier screw OA is rotatably arranged inside a tube-like container 04. This container 04 has on one end portion a toner entrance 04a and on the other end portion a toner exit 04b. 
The prior powder carrier screw OA includes a rotation shaft 010 and a coil 01 which is formed in a spiral form and carries toner and developer by using the rotation of the rotation shaft.
The rotation shaft 010 includes a driving shaft 02 to which a rotation driving force is input and a supporting shaft 03 having a one end supported by the driving shaft 02. The diameter of the driving shaft 02 is larger than that of the supporting shaft 03. The supporting shaft 03 is joined to the end portion of the coil 01.
One end of the driving shaft 02 projects from the container 04, and a gear 05 which inputs the rotation driving force is attached to that one end. The powder carrier screw OA rotates upon the input of the rotation driving force to the gear 05.
If the powder carrier screw OA rotates, the toner and the developer from the toner entrance 04a are pushed by a spiral coil face 01a formed on the rotation coil 01 to be moved, and are carried to the toner exit 04b. 
The supporting shaft 03 is joined to the end portion of the coil 01, so that the rotative force of the supporting shaft 03 is transferred to the end of the coil 01. Moreover, even if a large rotation load is generated in the coil 01, the rigidity of the coil 01 is maintained.
Next, the joining structure of the coil 01 and the driving shaft 02 of the prior powder carrier screw OA will be described. JP H11-223986A and the like describe that the coil 01 and the driving shaft 02 are joined by means of soldering and welding.
FIG. 9 illustrates an example in which the coil 01 and the driving shaft 02 are joined by soldering. In this soldering, it is necessary to apply solder 08 over a sufficient length along the spiral form of the coil 01, in order to enhance the joining strength.
Next, the joining structure of the driving shaft 02 and the supporting shaft 03 will be described. When joining the driving shaft 02 and the supporting shaft 03, as illustrated in FIG. 10, the driving shaft 02 and the supporting shaft 03 are joined by solder 09 in a state in which the supporting shaft 03 is fitted to a hole 02b (refer to FIG. 11) formed in the shaft center of the driving shaft 02, so that an increased joining strength can be obtained.
In this case, as illustrated in FIG. 11, the hole 02b to which the supporting shaft 03 is fitted is provided in the shaft center of the end face 02a on a side into which the coil 01 is inserted. The supporting shaft 03 and the end face 02a of the driving shaft 02 are joined by the solder 09 in a state in which the supporting shaft 03 is fitted to the hole 02b as illustrated in FIG. 10.
In this joining structure, it is necessary to apply the solder 09 over the entire circumference of the supporting shaft 03, in order to enhance the joining strength. In this case, as illustrated in FIG. 11, a spot facing portion 02c is provided in the entrance of the hole 02b of the driving shaft 02. The solder 09 enters into the spot facing portion 02e, so that an anchor effect can be obtained; thus, the joining strength can be further creased.
However, as illustrated in FIG. 9, in the prior powder carrier screw OA, the coil 01 and the driving shaft 02 are joined by the soldering. For this reason, the prior powder carrier screw OA has a problem in that the process which applies the solder 08 is complex, resulting in the deterioration in the productivity. Moreover, the operation environment is undesirable because flux is evaporated in the manufacturing. Furthermore, as illustrated in FIG. 12, if a spiral coil 011 having a narrow pitch is used, solder 08b fills in the spiral groove which is a path for carrying toner and developer, so that the carrying performance may be lost.
In addition, a heat treatment such as a nitriding treatment and a surface treatment may be applied to the driving shaft 02, in order to enhance abrasion resistance and corrosion resistance. When such a treatment is applied, the solder 08 is removed by the surface treatment, disenabling the joining. To combat this, the surface around the solder is shaved, but such an operation deteriorates a necessary characteristic such as corrosion resistance.
In the structure in which the coil 01 and the driving shaft 02 are joined by the soldering as described above, it is desired to improve the productivity and the operation environment. Moreover, it is difficult to firmly join the coil and the driving shaft after ensuring the carrying performance and the abrasion resistance.
Consequently, in order to solve the above problems, it is considered to use welding instead of using the soldering. When welding the coil 01 and the driving shaft 02, a condition which can weld the thicker driving shaft 02 is set as a general welding condition. However, by such a condition, the temperature of the thinner coil 01 is excessively increased, so that the coil 01 may be melted. For this reason, in fact, it is difficult to use the welding for the joining of the coil 01 and the driving shaft 02.
Moreover, as illustrated in FIG. 8, there may be a case in which a flange 02f provided in the driving shaft 02 has contact with the inner circumference of the container 04 for positioning, and the flange 02f rotates while having contact with the container 04 in the rotation. In such a configuration, if the carrying area of toner and developer continues just before the flange 02f, it is necessary to bring the leading end of the coil 01 into contact with the flange 02f for joining. In this case, the thinner flange 02f may be deformed or be damaged by the high heat of the welding, so that the positioning accuracy may be deteriorated. In addition, if the powder carrier screw OA rotates, the deformed flange 02f has contact with the container 04, which has a negative effect on the rotation. Consequently, it is also difficult to use the welding for the joining between the coil 01 and the driving shaft 02.
If the welding is used for the joining of the driving shaft 02 and the supporting shaft 03, where the difference between the diameter of the driving shaft 02 and the diameter of the supporting shaft 03 is large, the supporting shaft 03 may be melted by the high heat in the welding. For this reason, it is also difficult to use the welding for the joining of the driving shaft 02 and the supporting shaft 03.